A new genetic study shows even siblings with autism often have very different DNA mutations from one another— a finding that strengthens the evidence that autism is often just genetic bad luck.

The deep dive into the DNA of 170 people with autism spectrum disorder shows that more than 69 percent of brothers and sisters with autism had different DNA mutations underlying their disorders, Dr. Stephen Scherer of The Hospital for Sick Children in Toronto, Canada and colleagues found.

“It’s random mutation in these families. It just happens to be lightning striking twice,” Scherer said.

"It just happens to be lightning striking twice.”

The findings, published in the journal Nature Medicine, add to other research that shows “autism” is a broad term for a range of developmental disorders that are barely related to one another. The catchall name could be making it seem like one disorder is affecting many kids, when in fact it’s a range of conditions.

“It is largely just a random effect,” Scherer said. “Everybody in the population accumulates new mutations in the genome when they are conceived.”

Autism spectrum disorder can range from the mild social awkwardness, including Asperger’s syndrome, to profound mental retardation, debilitating repetitive behaviors and an inability to communicate. There’s no cure, but experiments with early treatment suggest it can help.

Autism is becoming more and more common among U.S. kids, and researchers don’t quite understand why. The last survey by the Centers for Disease Control and Prevention showed 2 percent of U.S. children have been diagnosed with an autism spectrum disorder — one in 68 kids.

Studies also show it’s clear that genetic mutations are responsible for a lot of it. Autism can run in families.

Scherer’s team set out to see if siblings with autism were inheriting some common pattern of mutations from their parents. They recruited 85 families where more than one child had been diagnosed, and sequenced everyone’s entire genome.

In general, brothers and sisters shared fewer than 30 percent of the mutations associated with autism, the researchers found. Instead, each child had his or her own unique set of genetic mutations causing the symptoms.

Maybe the families are just mutation prone? Scherer thought of this too, so they checked. But the kids did not have any more than the average number of new genetic mutations. Theirs just happened to be in the areas affecting the brain.

“We just think there are a lot of different targets,” Scherer said. “It’s a roll of the genetic dice.”

“It’s a roll of the genetic dice.”

Even that’s not surprising, Scherer added. “There are something like 25,000 genes in the human genome. A large number of them are involved in brain development,” Scherer said. It’s known there are at least 100 different genes associated with autism, and likely more.

It’s also clear that whatever is happening to these kids happens long before they’re born. The mutations affect every cell in the body, something only seen when it’s affecting the DNA at or soon after conception.

However, siblings who had the same symptoms often had the same mutations underlying those symptoms.

The team also found, however, that many of the kids had other disorders besides autism. “There are at least 100 different disorders that carry different names that have an autism component to them,” Scherer said.

The study also confirmed research that shows older fathers have a higher risk of having children with autism. Scherer said 76 percent of the new mutations in kids came from their dads probably because sperm is more mutation prone later in life.

The findings don’t surprise Valerie Hu, who studies the genetics of autism at the George Washington University School of Medicine and Health Sciences.

“It could be just a matter of luck, which genes are hit,” said Hu, who was not involved in the research.